[PATCH] asan, v3: Fix up handling of > 32 byte aligned variables with -fsanitize=address -fstack-protector* [PR110027]

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Mar 26, 2024 at 02:08:02PM +0800, liuhongt wrote:
> > > So, try to add some other variable with larger size and smaller alignment
> > > to the frame (and make sure it isn't optimized away).
> > >
> > > alignb above is the alignment of the first partition's var, if
> > > align_frame_offset really needs to depend on the var alignment, it probably
> > > should be the maximum alignment of all the vars with alignment
> > > alignb * BITS_PER_UNIT <=3D MAX_SUPPORTED_STACK_ALIGNMENT
> > >
> 
> In asan_emit_stack_protection, when it allocated fake stack, it assume
> bottom of stack is also aligned to alignb. And the place violated this
> is the first var partition. which is 32 bytes offsets,  it should be
> BIGGEST_ALIGNMENT / BITS_PER_UNIT.
> So I think we need to use MAX (BIGGEST_ALIGNMENT /
> BITS_PER_UNIT, ASAN_RED_ZONE_SIZE) for the first var partition.

Your first patch aligned offsets[0] to maximum of alignb and
ASAN_RED_ZONE_SIZE.  But as I wrote in the reply to that mail, alignb there
is the alignment of just a single variable which is the first one to appear
in the sorted list and is placed in the highest spot in the stack frame. 
That is not necessarily the largest alignment, the sorting ensures that it
is a variable with the largest size in the frame (and only if several of
them have equal size, largest alignment from the same sized ones).  Your
second patch used maximum of BIGGEST_ALIGNMENT / BITS_PER_UNIT and
ASAN_RED_ZONE_SIZE.  That doesn't change anything at all when using
-mno-avx512f - offsets[0] is still just 32-byte aligned in that case
relative to top of frame, just changes the -mavx512f case to be 64-byte
aligned offsets[0] (aka offsets[0] is then either 0 or -64 instead of either
0 or -32).  That will not help if any variable in the frame needs 128-byte,
256-byte, 512-byte ...  4096-byte alignment.  If you want to fix the bug in
the spot you've touched, you'd need to walk all the
stack_vars[stack_vars_sorted[si2]] for si2 [si + 1, n - 1] and for those
where the loop would do anything (i.e.
stack_vars[i2].representative == i2
&& TREE_CODE (decl2) == SSA_NAME
   ? SA.partition_to_pseudo[var_to_partition (SA.map, decl2)] == NULL_RTX
   : DECL_RTL (decl2) == pc_rtx
and the pred applies (but that means also walking the earlier ones!
because with -fstack-protector* the vars can be processed in several calls) and
alignb2 * BITS_PER_UNIT <= MAX_SUPPORTED_STACK_ALIGNMENT
and compute maximum of those alignments.
That maximum is already computed,
data->asan_alignb = MAX (data->asan_alignb, alignb);
computes that, but you get the final result only after you do all the
expand_stack_vars calls.  You'd need to compute it before.

Though, that change would be still in the wrong place.
The thing is, it would be a waste of the precious stack space when it isn't
needed at all (e.g.  when asan will not at compile time do the use after
return checking, or if it won't do it at runtime, or even if it will do at
runtime it will waste the space on the stack).

The following patch fixes it solely for the __asan_stack_malloc_N
allocations, doesn't enlarge unnecessarily further the actual stack frame.
Because asan is only supported on FRAME_GROWS_DOWNWARD architectures
(mips, rs6000 and xtensa are conditional FRAME_GROWS_DOWNWARD arches, which
for -fsanitize=address or -fstack-protector* use FRAME_GROWS_DOWNWARD 1,
otherwise 0, others supporting asan always just use 1), the assumption for
the dynamic stack realignment is that the top of the stack frame (aka offset
0) is aligned to alignb passed to the function (which is the maximum of alignb
of all the vars in the frame).  As checked by the assertion in the patch,
offsets[0] is 0 most of the time and so that assumption is correct, the only
case when it is not 0 is if -fstack-protector* is on together with
-fsanitize=address and cfgexpand.cc (create_stack_guard) created a stack
guard.  That is the only variable which is allocated in the stack frame
right away, for all others with -fsanitize=address defer_stack_allocation
(or -fstack-protector*) returns true and so they aren't allocated
immediately but handled during the frame layout phases.  So, the original
frame_offset of 0 is changed because of the stack guard to
-pointer_size_in_bytes and later at the
              if (data->asan_vec.is_empty ())
                {
                  align_frame_offset (ASAN_RED_ZONE_SIZE);
                  prev_offset = frame_offset.to_constant ();
                }
to -ASAN_RED_ZONE_SIZE.  The asan_emit_stack_protection code wasn't
taking this into account though, so essentially assumed in the
__asan_stack_malloc_N allocated memory it needs to align it such that
pointer corresponding to offsets[0] is alignb aligned.  But that isn't
correct if alignb > ASAN_RED_ZONE_SIZE, in that case it needs to ensure that
pointer corresponding to frame offset 0 is alignb aligned.

The following patch fixes that.  Unlike the previous case where
we knew that asan_frame_size + base_align_bias falls into the same bucket
as asan_frame_size, this isn't in some cases true anymore, so the patch
recomputes which bucket to use and if going to bucket 11 (because there is
no __asan_stack_malloc_11 function in the library) disables the after return
sanitization.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-04-11  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/110027
	* asan.cc (asan_emit_stack_protection): Assert offsets[0] is
	zero if there is no stack protect guard, otherwise
	-ASAN_RED_ZONE_SIZE.  If alignb > ASAN_RED_ZONE_SIZE and there is
	stack pointer guard, take the ASAN_RED_ZONE_SIZE bytes allocated at
	the top of the stack into account when computing base_align_bias.
	Recompute use_after_return_class from asan_frame_size + base_align_bias
	and set to -1 if that would overflow to 11.

	* gcc.dg/asan/pr110027.c: New test.

--- gcc/asan.cc.jj	2024-04-10 09:54:39.661231059 +0200
+++ gcc/asan.cc	2024-04-10 12:12:11.337978004 +0200
@@ -1911,19 +1911,39 @@ asan_emit_stack_protection (rtx base, rt
     }
   str_cst = asan_pp_string (&asan_pp);
 
+  gcc_checking_assert (offsets[0] == (crtl->stack_protect_guard
+				      ? -ASAN_RED_ZONE_SIZE : 0));
   /* Emit the prologue sequence.  */
   if (asan_frame_size > 32 && asan_frame_size <= 65536 && pbase
       && param_asan_use_after_return)
     {
+      HOST_WIDE_INT adjusted_frame_size = asan_frame_size;
+      /* The stack protector guard is allocated at the top of the frame
+	 and cfgexpand.cc then uses align_frame_offset (ASAN_RED_ZONE_SIZE);
+	 while in that case we can still use asan_frame_size, we need to take
+	 that into account when computing base_align_bias.  */
+      if (alignb > ASAN_RED_ZONE_SIZE && crtl->stack_protect_guard)
+	adjusted_frame_size += ASAN_RED_ZONE_SIZE;
       use_after_return_class = floor_log2 (asan_frame_size - 1) - 5;
       /* __asan_stack_malloc_N guarantees alignment
 	 N < 6 ? (64 << N) : 4096 bytes.  */
       if (alignb > (use_after_return_class < 6
 		    ? (64U << use_after_return_class) : 4096U))
 	use_after_return_class = -1;
-      else if (alignb > ASAN_RED_ZONE_SIZE && (asan_frame_size & (alignb - 1)))
-	base_align_bias = ((asan_frame_size + alignb - 1)
-			   & ~(alignb - HOST_WIDE_INT_1)) - asan_frame_size;
+      else if (alignb > ASAN_RED_ZONE_SIZE
+	       && (adjusted_frame_size & (alignb - 1)))
+	{
+	  base_align_bias
+	    = ((adjusted_frame_size + alignb - 1)
+	       & ~(alignb - HOST_WIDE_INT_1)) - adjusted_frame_size;
+	  use_after_return_class
+	    = floor_log2 (asan_frame_size + base_align_bias - 1) - 5;
+	  if (use_after_return_class > 10)
+	    {
+	      base_align_bias = 0;
+	      use_after_return_class = -1;
+	    }
+	}
     }
 
   /* Align base if target is STRICT_ALIGNMENT.  */
--- gcc/testsuite/gcc.dg/asan/pr110027.c.jj	2024-04-10 12:01:19.939768472 +0200
+++ gcc/testsuite/gcc.dg/asan/pr110027.c	2024-04-10 12:11:52.728229147 +0200
@@ -0,0 +1,50 @@
+/* PR middle-end/110027 */
+/* { dg-do run } */
+/* { dg-additional-options "-fstack-protector-strong" { target fstack_protector } } */
+/* { dg-set-target-env-var ASAN_OPTIONS "detect_stack_use_after_return=1" } */
+
+struct __attribute__((aligned (128))) S { char s[128]; };
+struct __attribute__((aligned (64))) T { char s[192]; };
+struct __attribute__((aligned (32))) U { char s[256]; };
+struct __attribute__((aligned (64))) V { char s[320]; };
+struct __attribute__((aligned (128))) W { char s[512]; };
+
+__attribute__((noipa)) void
+foo (void *p, void *q, void *r, void *s)
+{
+  if (((__UINTPTR_TYPE__) p & 31) != 0
+      || ((__UINTPTR_TYPE__) q & 127) != 0
+      || ((__UINTPTR_TYPE__) r & 63) != 0)
+    __builtin_abort ();
+  (void *) s;
+}
+
+__attribute__((noipa)) int
+bar (void)
+{
+  struct U u;
+  struct S s;
+  struct T t;
+  char p[4];
+  foo (&u, &s, &t, &p);
+  return 42;
+}
+
+__attribute__((noipa)) int
+baz (void)
+{
+  struct W w;
+  struct U u;
+  struct V v;
+  char p[4];
+  foo (&u, &w, &v, &p);
+  return 42;
+}
+
+int
+main ()
+{
+  bar ();
+  baz ();
+  return 0;
+}


	Jakub